Process for Purifying Compressed Air, Compressed Air Purification Appartus and Motor Vehicle Having Such an Apparatus

ABSTRACT

In a compressed air purification apparatus for motor vehicles, compressed air under static pressure in a pressure conduit ( 3 ) is firstly freed of impurities such as hydrocarbon compounds and oil products and subsequently dried. To ensure effective freeing of the compressed air of impurities and water vapour and provide a compressed air purification apparatus which can be used with simple means in motor vehicles, the invention provides for the compressed air being brought, before purification, to such a temperature that the gaseous impurities in the compressed air are condensed and the water present in the compressed air is dissolved as vapour.

The invention generally relates to a method for processing compressed air for one or more pneumatic consumers, of the type specified in the preamble of patent claim 1, to a compressed air processing device as per the preamble of patent claim 6, and to a motor vehicle having a compressed air processing device of said type.

To supply compressed air to pneumatic consumers such as compressed-air brakes of motor vehicles, compressed air supply devices are known in which the compressor compresses intake air and supplies the air via a pressure line to a storage vessel, from which the air is extracted as required. In motor vehicle applications, the humidity of the compressed air is undesirable. DE 10 2006 035 772 A1 discloses a compressed air processing device for motor vehicles in which a dehumidification device is provided in the pressure line such that the compressed air is dried before entering the storage vessel.

The compressed air is, however, also often contaminated with damaging matter from the intake air and with oil products and oil decomposition products from the compression process of the compressor, which is usually lubricated with oil. Such contaminants—which are present in the compressed air in addition to the water—will for simplicity hereinafter be referred to as oil products. A large proportion of the oil products is present as aerosol, which moves with the compressed air and can therefore pass into the entire compressed air system. There, the oil products can lead to damage, for example as a result of the swelling, sticking or decomposition of seal elements. To protect the compressed air device, and the compressed air units connected thereto, against contamination with oil products, the compressed air in the pressure line is generally purified of oil products.

Solutions for the processing of the compressed air have already been proposed in which the purification of the oil takes place before the drying. Here, it is known for a separate filter, for example a coalescence filter or an oil separator, to be provided as a device for oil purification. However, it has proven to be disadvantageous that separated oil products can be vented into the environment. Oil separators operating according to the cyclone principle separate only aerosols of a particular size and therefore, in many applications, do not permit adequate purification performance. In particular, however, the filtering of the compressed air before it flows through the dryer has the effect that the device for oil purification is impinged on both with oil products and also with water. The filter element therefore separates a considerable amount of liquid as a mixture of oil and water, which must be disposed of in considerable quantities, which in turn necessitates frequent servicing of the filter, and under some circumstances impairs the function thereof.

It has also already been proposed that oil purification first be carried out downstream of the drying agent. DE 103 135 75 A1 discloses a cartridge device for an air dryer, in which the means for removing damaging chemical compounds, in particular hydrocarbon compounds, is positioned downstream of the drying agent with respect to the flow direction of the air. The fact that filtration first takes place downstream of the drying agent, however, often leads to excessive contamination of the drying agent, which significantly reduces the drying performance.

The invention is based on the objects of providing a compressed air processing device and a method for generating compressed air for a pneumatic consumer, which ensure that the compressed air is purified of oil products and water vapor in an effective manner without separated oil products being vented into the environment, and which can be implemented in motor vehicles using simple means.

These objects are achieved according to the invention by means of a method having the features of patent claim 1, by means of a compressed air processing device having the features of patent claim 6, and by means of a motor vehicle having the features of patent claim 15.

According to an embodiment of the invention, in the case of processing of the compressed air in which the compressed air is purified of oil products in a first step and is subsequently dried, the compressed air, before the purification for the removal of oil, is brought to a temperature at which the hydrocarbon compounds condense and the water mass contained in the compressed air is dissolved as steam. Here, the compressed air is purified of oil by filtration at a selected temperature at which the oil products are completely condensed and are therefore present as aerosols or in liquid form, which can be easily filtered, whereas the water is still dissolved, as water vapor, in the compressed air and passes through the filter. The water vapor may then be removed from the compressed air in an effective manner using simple means during the subsequent drying.

The temperature must be selected to be so high that, allowing for the ambient conditions and the respective compression, the humidity of the compressed air of the compressed air supply system still remains dissolved in the compressed air as water vapor. On the other hand, the temperature must be selected to be so low that the oil products are completely or almost completely condensed. In the case of an ambient temperature of for example −40° C., an inlet temperature of for example −15° C. during the purification for the removal of oil is regarded as an advantageous temperature value. At an ambient temperature of 20° C., the inlet temperature may advantageously be 70° C., and at an ambient temperature of 40° C., the inlet temperature may be 100° C. The pressure line and the devices and assemblies possibly arranged therein are coordinated with one another such that the compressed air temperature provided according to the invention, at which oil is condensed and the entrained water remains dissolved as steam, prevails at the inlet of the processing device, that is, upstream of the device for removing oil products with respect to the throughflow direction.

During the separation or filtering of oil according to the invention, and the water separation which takes place separately therefrom, only a relatively small amount of oil products is present, which is easier to handle and simpler to dispose of. The water, which is generally separated in larger quantities, is clean and can be vented into the environment. Furthermore, as a result of the processing according to the invention, the dryer or the drying agent stored in the dryer is reliably protected. If the drying unit is formed as a cartridge with drying agent stored therein, the servicing intervals of the cartridge are considerably lengthened, or the cartridge need no longer be exchanged at all. Since the processing of compressed air according to the invention is extremely efficient with regard to the oil separation, coarse filtering in the drying agent cartridge can be dispensed with entirely, such that components of simpler design are adequate for complete processing. Furthermore, with the compressed air purification for the removal of oil according to the invention, the entrained oil is completely collected and disposed of, such that no oil products pass into the environment. In this way, allowance is made for the increasing demands on motor vehicles with regard to environmental protection.

The temperature level according to the invention leading to the condensation of only the oil aerosols can be set using simple means, allowing for the compressor characteristic and the compression, by means of the design of the pressure line 4. For this purpose, the line length, the line cross section, the thermal conductivity and capacity of the pressure line are correspondingly selected, and the profile thereof when laid in the vehicle is selected such that the pressure line generates the desired compressed air temperature.

A device that influences the temperature of the compressed air is advantageously arranged in that portion of the pressure line situated upstream of the device for removing oil with respect to the throughflow direction. Through the use of the device, the temperature at the inlet of the filter unit is set to the level according to the invention, at which the oil products are present as aerosols and the water vapor remains dissolved in the compressed air. Since the compressed air is heated when compressed by a compressor, the unit that influences the temperature of the compressed air is a cooler, which sets the compressed air to the level according to the invention. For this purpose, in a particularly advantageous embodiment of the invention, a control or regulating unit is provided that, in order to regulate or control the temperature, actuates the cooler as a control element.

In one advantageous embodiment of the invention, the compressed air is additionally cooled after the purification, in order that the subsequent drying can take place in an effective manner. Firstly, during the cooling process, water is condensed and can be directly vented as a liquid. Secondly, the further drying process (lowering of the dew point) is more effective the lower the temperature at which it takes place. If, in the case of an ambient temperature that lies considerably above the freezing point, the inlet temperature is lowered as close as possible to the ambient temperature, whereas in the case of a low ambient temperature, an inlet temperature of 25 . . . 30 K above ambient temperature is expedient, freezing of the dryer as a result of the formation of ice is prevented.

The purification of the compressed air for the removal of oil advantageously takes place by filtration, though other processing techniques may alternatively be used, for example the provision of an oil separator. As a filter, use may advantageously be made of a coalescence filter, which is a cost-effective filter option. The coalescence filter may expediently be part of an air processing unit (APU), such that a compact component is provided, into which also the drying unit and if appropriate further components are integrated according to the performance capability of the APU. The APU combines the purification for the removal of oil, the dryer and also the associated cooling in a compact housing, such that the number of components of the pressure supply device is considerably reduced. The compact design meets the increasing requirements for use in motor vehicles, where there is often only a very small amount of installation space for accommodating the devices to be supplied with compressed air. Furthermore, simple and fast installation is possible as a result of the reduced number of components in the air processing unit.

The efficiency of the separation, according to the invention, of the purification of the compressed air for the removal of oil and the subsequent water separation permits the use of components that can be of simple construction in comparison with the conventionally used assemblies and that can be designed exclusively with regard to their respective purpose.

A design of the oil filter as a cartridge has the advantage, aside from being easy to handle and install, that only the oil filter needs to be exchanged at regular intervals.

In a further advantageous embodiment of the invention, a shut-off valve is situated in the pressure line between the filter unit and drying unit, which serves to maintain a pressure in the portion from the compressor to the shut-off valve when the air dryer of the drying unit is vented for the purpose of regenerating the drying agent. In this way, firstly, a situation is prevented in which, during the venting process, contaminants from the filter unit, that is, the oil filter or oil separator, are carried along into the downstream line portion as a result of the high flow speeds occurring during the process. Secondly, during a regeneration, the pressure at the filter unit is maintained, and in this way energy is saved for restoring the pressure level after the regeneration. The shut-off valve may be externally controlled by an electronic air processor, or may also be of self-controlling design.

Exemplary embodiments of the invention will be explained in more detail below on the basis of the drawings, in which:

FIG. 1 is a fluid-circuit diagram of a compressed air processing device,

FIG. 2 is a fluid-circuit diagram of an alternative design of a compressed air processing device,

FIG. 3 is a fluid-circuit diagram of a further embodiment according to the invention of a compressed air processing device,

FIG. 4 is a graphic illustration of the profile with respect to time of the inlet temperature of the compressed air before the purification for the removal of oil.

In the drawing figures and in the following description, the same reference numerals are used in each case for identical components and devices.

FIG. 1 shows a compressed air supply device 1 for a motor vehicle, which comprises a compressor 2 that sucks in and compresses ambient air and that is connected via a pressure line 3 to a storage vessel 4. Assemblies (not illustrated) of a motor vehicle are connected to the storage vessel 4 and, as consumers, are supplied with compressed air from the storage vessel. The compressor 2 is driven by an engine 5, which may for example be the drive engine of the motor vehicle.

A device 6 for processing the compressed air is provided in the pressure line 3, the device having a filter unit 7 and a drying unit 9 situated downstream of the filter unit 7 with respect to the throughflow direction 8 of the pressure line 3. The filter unit 7 comprises a coalescence filter 10, which is arranged in the pressure line 3, for collecting contaminants, in particular oil products. Here, a vessel 11 for collecting the filtered-out oil is assigned to the filter 10. The coalescence filter 10 accumulates aerosols and also oil products present in liquid form. Liquid oil products pass either directly from the oil-lubricated compressor into the compressed air or condense on the wall of the pressure line. The pressure line is equipped with a bypass line (bypass) for the coalescence filter 10, and a check valve 12 is arranged in the bypass. The check valve 12, in a known manner, opens the bypass when the response pressure is reached, such that the pressure level at the coalescence filter 10 is reduced.

The compressed air that has already been purified of oil aerosols is dried in the drying unit 9. In the exemplary embodiment shown, the drying unit 9 has an air dryer 13 arranged in the pressure line 3 and has a drain valve, wherein the condensed and separated entrained water from the compressed air is discharged via an outlet 14. Within the drying unit 9, a check valve 15 is provided in the pressure line 3 between the air dryer 13 and the storage vessel 4, and maintains the working pressure in the storage vessel 4.

The pressure line 3, in its portion between the compressor 2 and the filter unit 7, is configured such that the compressed air at the inlet 16 of the compressed air processing device 6 is brought to such a temperature that the entrained oil in the compressed air is condensed, and the water mass also contained in the compressed air is dissolved as steam. In this way, the oil products of the compressed air are filtered out in the filter unit 7, whereas the water vapor flowing through is subsequently removed from the compressed air in the drying unit 9. The temperature of the compressed air in the pressure line 3 is higher than the ambient temperature owing to the compression in the compressor 2. The temperature level according to the invention that leads to the condensation of only the oil aerosols can be set, allowing for the compressor characteristic and the compression, by means of the design of the pressure line 3. For this purpose, the line length, the line cross section, the thermal conductivity and capacity of the pressure line are correspondingly selected, and the profile thereof when laid in the vehicle is selected such that the pressure line generates the desired compressed air temperature. The check valve 12 in the filter unit 7 limits the pressure at the inlet 16 of the compressed air processing device 6, and has a contributory effect in ensuring a continued supply of compressed air to the consumers even in the event of a blockage of the filter. In the event of a blockage, the pressure at the inlet 16 rises. When a predetermined pressure value is reached, the check valve 12 opens and opens up the bypass.

To obtain effective water separation in the drying unit 9, a cooler—not illustrated—is provided in the pressure line 3 between the filter unit 7 and the drying unit 9, which promotes the condensation of the water vapor in the line section before the inlet into the drying unit 9.

The coalescence filter 10 may be designed as a cartridge, such that an easily exchangeable component is provided. Here, the filter cartridge may be a constituent part of an air processing unit (APU), which is of compact construction and which also comprises the air dryer and the storage vessel 4.

A shut-off valve 17 is arranged in the pressure line 3 between the filter unit 7 and the drying unit 9, which serves to keep the portion from the compressor to the shut-off valve 17 under pressure when the air dryer 13 is vented for the purpose of regenerating the drying agent.

FIG. 2 shows an alternative design of the filter unit 18, which, in the illustration shown in FIG. 1, may be used instead of the filter unit 7 provided therein. In the filter unit 18 according to FIG. 2, in each case one cooler 19 and 20 is arranged upstream and downstream of the coalescence filter 10 with respect to the throughflow direction 8 of the pressure line 3, which lower the temperature of the compressed air flowing through them. Each of the coolers 19, 20 is assigned in each case one bypass of the pressure line 3, said bypasses being shut off in each case by a check valve 21 and 22. The check valves 21, 22 open in the throughflow direction 8 if the respective cooler 19, 20 becomes blocked, for example as a result of icing. The cooler 19 situated upstream of the coalescence filter 10 with respect to the throughflow direction 8 lowers the temperature of the compressed air, which is compressed and thereby simultaneously heated by the compressor, to the temperature level according to the invention at which oil products in the compressed air condense but the water vapor remains dissolved owing to the still adequately high temperature level. The cooler 20 arranged downstream of the coalescence filter 10 with respect to the throughflow direction 8 lowers the temperature level of the compressed air, which has been purified of oil products, such that after the oil filtration, the water vapor is rapidly condensed, and drying takes place in a highly effective manner. This lowering amounts to for example 25K. Before entering the drying unit 9 (FIG. 1), which is not illustrated here, downstream of the filter unit 18, the compressed air contains only moisture, the moisture being separated in the drying unit 9, such that the drying unit is protected against damaging oil products.

The oil products filtered out of the coalescence filter 10 are collected in the storage vessel 11 and can thus be disposed of in a simple manner at regular intervals. In the case of pressure supply devices in motor vehicles, the collection of the oil products firstly permits the use even of compressors that release oil products and oil decomposition products into the compressed air during their compression process. The products can specifically be completely collected as a result of the oil filtration according to the invention, and an escape of the products into the environment can be consistently counteracted. Secondly, the complete collection of the oil products and the prevention of an escape into the environment already makes allowance now for the possible legal restrictions on the overall pollutant emissions of a vehicle, and not only the exhaust-gas emissions thereof.

FIG. 3 shows a filter unit 27 for processing compressed air for an electronically controllable compressed air supply device. The compressed air supply device is assigned an electronic control unit 28, which controls the processing of the compressed air in the pressure line 3. The filter unit 27 is arranged, in the manner shown in FIG. 1 and described correspondingly, in the pressure line 3 between the compressor and the drying unit. The filter unit 27 comprises a coalescence filter 10, to which is assigned, in the manner already described, a collecting vessel 11 for collecting the separated oil products. The filling level of the collecting vessel 11 is detected by a liquid level sensor 29. In the exemplary embodiment shown, the liquid level sensor 29 is connected to the electronic control unit 28 via a signal line 30, such that the control unit 28 can detect the filling level of the storage vessel and, if required, display a prompt to empty the vessel 11.

Coolers 19, 20 are arranged in each case upstream and downstream of the coalescence filter 10 with respect to the throughflow direction 8 of the pressure line 3. Both coolers 19, 20 are assigned, similarly to the embodiment as per FIG. 2, a bypass line of the pressure line 3. In each case one control valve 31, 32 is arranged in the bypass lines, the control valves being actuated by the control unit 28 via signal lines 30. In the exemplary embodiment shown, the control valves 31, 32 are designed as 2/2 directional control valves, such that, as a function of the switching state of the control valves 31, 32, the control unit 28 can conduct the compressed air as required through the respective cooler 19, 20 or, bypassing the cooler, through the respective bypass lines. It is alternatively possible, for example, for valves that automatically switch in a temperature-dependent manner to be used instead of the externally actuated control valves 31, 32.

The control valve 31 situated upstream with respect to the throughflow direction 8 is operated by the control unit 28 as a control element for controlling the compressed air temperature upstream of the coalescence filter 10. To set the compressed air temperature according to the invention, in the case of which there prevails at the inlet of the coalescence filter 10 a temperature at which the oil products are condensed but the water vapor remains dissolved in the air flow, the inlet temperature upstream of the filter 6 is measured by means of a temperature sensor 33 as a control variable and is adjusted to the predefined setpoint value by activation and deactivation of the cooling. The activation of the cooling takes place in the present exemplary embodiment by closing the control valve 31, such that the compressed air to be purified is forced through the operating cooler 19. For the deactivation of the cooling, the control valve 31 is correspondingly switched by the control unit 28 into a pass-through position. As an alternative to control, regulation of the inlet temperature of the coalescence filter 10 may also be provided.

The cooler 20 situated downstream of the filter 10 is used by the control unit 28 in a corresponding way. To set a predefined outlet temperature of the filter unit 27, the control unit detects the outlet temperature of the second cooler 20 by means of a temperature sensor 34 arranged downstream of the cooler 20. The desired outlet temperature, at which effective drying of the compressed air is obtained, is set by corresponding actuation for opening/closing the control valve 32 of the second cooler 20.

In the region of the air inlet of the filter 10, that is, between the intermediate cooler 19 and the filter 10, a pressure sensor 35 is provided in addition to the temperature sensor 33. The measurement signal from the pressure sensor 35 is supplied to the control unit 28. The measurement, which is possible by means of the signal of the pressure sensor 35, of the back pressure upstream of the filter 10 may be used to check for correct functioning of the oil filtration or also for determining the filter exchange interval. Alternatively, the pressure sensor 35 or a further pressure sensor is arranged upstream of the intermediate cooler 19.

The filter unit 27 is combined with a drying unit 9 (FIG. 1) in a compact unit that is controlled by the control unit 28. This is, therefore, referred to as an electronically controlled air processing unit (E-APU). Such an air processing unit may also comprise further components, such as for example a protective valve for protection of the different compressed air circuits.

FIG. 4 shows, on the basis of a graphic illustration of the profile of the temperature of the compressed air upstream of the oil filter, the functioning of the control of the compressed air processing by means of the temperature of the compressed air. The graphic temperature profile is denoted by T-Act. In addition to the temperature curve, the diagram of FIG. 4 also contains a graphic illustration of the switching state of the cooling. The cooling is activated and deactivated by the control unit in the manner described with regard to FIG. 3, so as to yield the rectangular curve profile of the cooler function illustrated in FIG. 4. Those portions of the cooler curve that are situated on the X axis correspond here to the deactivated state. Those portions of the cooler curve that run between the time segments with deactivated cooling correspond to the activated state Cooler (on). In the time segments with activated cooling, the temperature T-Act of the compressed air upstream of the oil filter falls and is kept within the temperature window in which the oil products condense. To keep the temperature of the compressed air in an interval in which the oil products condense but the water vapor remains dissolved in the air flow, the boundary values of the temperature interval are predefined to the controller. If the temperature T-Act of the compressed air when the cooling is activated reaches the lower boundary value T-Setpoint-Min, the control unit switches the control valve 31 (FIG. 3) into the pass-through position, such that the cooling is deactivated and the temperature T-Act rises. When the maximum value T-Setpoint-Max is reached, the control valve 31 is closed again, such that the compressed air flows through the cooler 19 and is cooled again.

The second cooler 20 downstream of the coalescence filter 10 may also be controlled in a similar way to that illustrated in FIG. 4. Here, a lower setpoint temperature is predefined and kept in the likewise predefined temperature window by activation and deactivation of the cooling.

All of the features specified in the description of the figures, in the claims and in the introductory part of the description may be used both individually and also in any desired combination with one another. The invention is therefore not restricted to the described or claimed combinations of features. In fact, all combinations of features should be regarded as being disclosed. 

1. A method for processing compressed air for one or more pneumatic consumers, wherein the compressed air in a pressure line (3) is firstly purified of contaminants, such as hydrocarbon compounds and oil products, and subsequently dried, characterized in that the compressed air, before the purification for the removal of contaminants, is brought to a temperature (T-Act) which is such that the contaminants present in gaseous form condense and the water mass contained in the compressed air is dissolved as steam.
 2. The method as claimed in claim 1, characterized in that the temperature (T-Act) of the compressed air is controlled or regulated before the purification.
 3. The method as claimed in claim 1 or 2, characterized in that the compressed air is cooled before the purification.
 4. The method as claimed in claim 2 or 3, characterized in that the temperature (T-Act) of the compressed air before the purification is controlled or regulated by virtue of a cooler (19) for the compressed air being activated and deactivated.
 5. The method as claimed in one of the preceding claims, characterized in that the compressed air is cooled after the purification and before the drying.
 6. A device for processing compressed air, having a pressure line (3) which conducts compressed air and in which a device (7, 18, 27) for removing contaminants, such as hydrocarbon compounds and oil products, and a drying unit (13) are arranged in series in a throughflow direction (8), characterized in that the pressure line (3), in its portion situated upstream of the device (7, 18, 27) for removing contaminants with respect to the throughflow direction (8), is designed such that the contaminants present in gaseous form condense and the water mass contained in the compressed air is dissolved as steam.
 7. The compressed air processing device as claimed in claim 6, characterized in that the pressure line (3) is designed, with regard to its line length and/or the line cross section and/or the thermal conductivity and capacity and/or its profile, such that the contaminants present in gaseous form condense and the water mass contained in the compressed air is dissolved as steam.
 8. The compressed air processing device as claimed in claim 6 or 7, characterized in that a unit (19) which influences the temperature (T-Act) of the compressed air is arranged in the pressure line (3), in that portion thereof which is situated upstream of the device (7, 18, 27) for removing contaminants with respect to the throughflow direction (8).
 9. The compressed air processing device as claimed in claim 8, characterized in that the unit which influences the temperature of the compressed air (T-Act) is a cooler (19).
 10. The compressed air processing device as claimed in one of claims 7 to 9, characterized in that a shut-off valve (17) is arranged in the pressure line (3) between the filter unit (7) and drying unit (9).
 11. The compressed air processing device as claimed in one of claims 7 to 10, characterized in that a regulating or control unit for the temperature (T-Act) of the compressed air is provided, which regulating or control unit interacts with the cooler.
 12. The compressed air processing device as claimed in one of claims 7 to 11, characterized in that the device (7, 18, 27) for removing contaminants comprises a coalescence filter (10).
 13. The compressed air processing device as claimed in one of claims 7 to 12, characterized in that the device (7, 18, 27) for removing contaminants is designed as a cartridge.
 14. The compressed air processing device as claimed in one of claims 7 to 13, characterized in that the device (7, 18, 27) for removing contaminants and the drying unit (13) form a structural unit.
 15. A motor vehicle having a compressed air processing device as claimed in one of claims 6 to 13, which compressed air processing device can be operated using a method as claimed in one of claims 1 to
 5. 